Synthesis Of Noble Metal-based Nanocomposites And Their Application In Gas Treatment

With the advent of the industrialization era,air pollution problems have become increasingly serious.The emissions of automobile exhaust and industrial waste gas have caused the levels of carbon monoxide(CO)and volatile organic compounds(VOCs)such as formaldehyde and toluene to exceed standards,which are great hazardous to the ecological environment and human health.The most attractive approach for the treatment of the pollutions are catalytic oxidation to some harmless small molecular substance due to the high removal efficiency,low cost,and no secondary pollution.So far,noble metal-based nanocomposites have been extensively investigated in the field of environmental restoration due to their unique advantages,such as reducing the amount of noble metals to save costs and high catalytic activity.However,there are still some problems and difficulties in previous research reports:1 how to choose suitable substrate materials and control its morphology and surface modification;2.how to design a simple and easy method to maintain high stability and high dispersion of noble metal nanoparticles;3.how to design the multifunctionalization of catalytic materials(effective combination of adsorption,detection and catalysis)to improve the application value;4.how to achieve the immobilization and recycling of powder catalyst.Therefore,it is of great significance to research and design efficient,stable,reusable and versatile catalytic materialsBased on the mentioned problems,using the CO,formaldehyde,and toluene as model pollutants,aiming to build a high-performance,multifunctional noble metal-based nanocatalyst by simple and feasible methods,that is,noble metals(Au,Ru,Pt,Pd)as the main catalytic material,porous materials with large surface area(such as mesoporous carbon spheres,mesoporous silica,graphene,etc.)or metal oxides(such as Co3O4,CeO2,NiCo2O4,etc.)as substrate materials.Adopting effective methods to synthesize highly dispersed and stable noble metal nanoparticles,complete the morphology adjustment of the substrate materials,achieve catalyst surface modification,immobilization,and functionalization,increase the surface area,improve catalytic performance and catalytic rate,and broaden its application in the field of environmental restoration.The main work is carried out in the following aspects:(1)Through simple emulsion polymerization and post-heating treatment methods,hollow porous carbon nanospheres(HPCN)were successfully synthesized,the pores of the carbon nanospheres were used as containers to adsorb Au3+,and then the obtained HPCN contained Au3+was put in the reactor containing the catalytic atmosphere,utilizing the reducibility of CO to complete the loading of gold nanoparticles and continue to be used for catalytic oxidation of CO.In the synthesis process,the preparation of the catalyst and the removal of CO formed a continuous process,optimizing the synthesis steps.At the same time,the gold nanoparticles were confined in the pores,achieving the effect of uniform distribution and size,with an average diameter of 2.2 nm,which can achieve 62%conversion at 340℃.Noble metal nanoparticles are fixed by the channels of the substrate,which solves the problem of easy agglomeration,and provides a feasible and effective method for preparing highly dispersed and stable noble metal nanoparticles(2)To achieve high efficient catalysis at room temperature,the ruthenium nanoparticles were selected to be supported on graphene and further to form an aerogel,then,a tunable metal-organic framework(HKUST-1)coating is applied to the composite by using layer-by-layer self-assembly methods.The obtained Ru nanoparticles have a uniform distribution and size,with an average diameter of about 5nm,which provides a cornerstone for room temperature catalysis;the open macroporous structure of graphene aerogels provide pathways for the access and diffusion of gas molecules;the selected HKUST-1 has an adsorption ability for CO,which can increase the instantaneous concentration of CO around the catalyst,thereby improving the reaction rate.In this work,a functional composite material with integrated adsorption and catalysis was successfully prepared.After pretreatment,CO can be completely removed at room temperature and no significant decrease in activity within 48 hours,which provide a good application prospect in the treatment of CO pollution.(3)CO is colorless and odorless,and it is difficult to be noticed after leakage.Therefore,the visualization of CO is imminent.The Pt nanoparticles were loaded on the hollow mesoporous silica spheres(Pt/HMSs)by a vacuum-assisted incipient wetness impregnation method.And then combined it with a silica gel containing an adsorbed chromogenic probe(binuclear rhodium complex,BRC),the BRC has good selectivity and color response to CO in the air.A quickly and eye-readable color change at the CO atmosphere was observed,and the color change is completely reversible,which synthesizes a functional composite material integrating detection and catalysis.The Pt nanoparticles were encapsulated in the pores,with uniform size and distribution,and the Pt/HMSs composites possess good stability and catalytic activity,which can achieve complete conversion of CO at 150℃.Furthermore,the active sites of Pt/HMSs for CO oxidation through DFT calculations were explored,which provided the possibility of designing highly active catalysts and increased its application value in exhaust treatment.(4)In addition to CO,formaldehyde is also a major aspect of indoor air pollution According to the research,there is an interaction between metal oxides and noble metals,which can improve catalytic performance,so the gold nanoparticles were loaded on cerium and cobalt oxides(Au-CexCoy)for the treatment of formaldehyde.The obtained catalysts possess a well-defined structure of ordered arrays of nanotubes,which significantly increases the surface area;the synthesized Au nanoparticle has uniform size and distribution;the catalysts were combined with graphene to form an aerogel(Au-CexCoy/GA),which realizes the immobilization of the powder catalyst.To investigate the synergy between CeO2 and Co3O4,the different ratios of CeO2 to Co3O4 on catalytic performance were compared.The results reveal that when the molar ratio of Ce/Co is 3:1,the obtained catalyst shows excellent performance for catalytic oxidation of formaldehyde due to promote the migration of oxygen species and activate of Au,which can reach 100%conversion at 60℃,and no significant decrease in activity within 48 hours(5)The two metal ions in the spinel-type oxide are located at tetrahedral and octahedral positions,respectively,which can improve the catalytic performance as a result of electron exchange.Therefore,hollow-structure nickel cobaltate nanospheres(h-NiCoOx)were prepared by the hard template method,and the palladium nanoparticles were further supported on the surface(Pd/h-NiCoOx)to study the catalytic performance for toluene.The obtained h-NiCoOx possesses a large surface area,abundant surface hydroxyls,and a small number of oxygen vacancies,which make the h-NiCoOx show excellent performance in catalytic toluene,100%conversion of 500 ppm toluene can be achieved at 250℃;the incorporation of Pd nanoparticles can further lower the activation energy and temperature,improve the reaction activity.The temperature at which toluene is completely converted is further reduced to 190℃.This work optimizes the substrate material through morphology adjustment and surface modification,and studies the effects of surface hydroxyls and surface area on the catalytic activity,providing a feasible method for the rapid synthesis of highly active substrate.